1. Field of the Invention
The present invention relates to a control device for an air-fuel ratio sensor, and more particularly, to a control device for an air-fuel ratio sensor which detects an impedance of an air-fuel ratio sensor device such as an oxygen concentration detecting device for rapidly and accurately detecting the air-fuel ratio of exhaust air from an internal combustion engine. The control device determines a fault or activation state of the air-fuel ratio sensor based on the detected impedance, calculates the air-fuel ratio from an output value of the air-fuel ratio sensor and corrects a target temperature to which the air-fuel ratio sensor device is to be heated by energizing a heater so that an activation state of the air-fuel ratio sensor device is maintained.
2. Description of the Related Art
In a recent trend to control air-fuel ratio of engines, an air-fuel ratio sensor and catalyst are disposed in the exhaust system of the engine and feedback control is performed to set an exhaust air-fuel ratio detected by the air-fuel ratio sensor to a target air-fuel ratio, for example, a stoichiometric air-fuel ratio to maximize purification of harmful exhaust components (HC, CO, NOx and the like) by the catalyst. Limit current type devices for detecting oxygen concentration have generally been employed as air-fuel ratio sensors to output a limit current proportional to the concentration of oxygen contained in the exhaust gas. The limit current type oxygen concentration detecting device detects the air-fuel ratio of the exhaust based on the oxygen concentration widely and linearly. It is useful for improving the air-fuel ratio control accuracy and controlling the air-fuel ratio of the exhaust to a target air-fuel ratio in a wide range from rich to the theoretical air-fuel ratio (stoichiometric) and further to lean.
The aforementioned oxygen concentration detecting device must be kept activated to maintain the air-fuel ratio detecting accuracy. In order to activate the device as soon as possible after a cold engine start, such devices are usually heated by energizing a heater provided therein at the startup of the engine. Further, the heater is controlled to maintain the activation state.
FIG. 19 is a diagram showing a correlation between the temperature of the oxygen concentration detecting device and the impedance thereof. There is a correlation indicated by a bold line of FIG. 19 between the temperature of the aforementioned oxygen concentration detecting device (hereinafter referred to as a device) and impedance, that is, the impedance of the device damps with a rise in the device temperature. In conjunction with this relation, during the heater energizing control, the impedance of the device is detected to derive the device temperature, and feedback control is carried out to set the device temperature to a desired activation temperature, for example, 700.degree. C. For example, when the impedance Zac of the device is equal to or greater than a device impedance 30.OMEGA. (Zac&gt;30) corresponding to the initial control device temperature of 700.degree. C., that is, when the device temperature is less than or equal to 700.degree. C., the heater is energized. When the Zac is smaller than 30.OMEGA. (Zac&lt;30), that is, when the device temperature exceeds 700.degree. C., the heater is de-energized. As a result, the device temperature is maintained equal to or greater than the activation temperature of 700.degree. C. so as to maintain the activation state. Further, when the heater is energized, duty control is carried out to obtain an energization amount required to eliminate deviation of the device impedance (Zac-30) from the target value, and to energize for the required power supply.
For example, as disclosed in Japanese Patent Application Laid-Open No. HEI 9-292364, when detecting the impedance of the aforementioned oxygen concentration detecting device, an alternating voltage of a single frequency required for detecting a temperature of such device is applied to detect the impedance. By applying a voltage of this frequency, a resistance of the electrolytic portion of the device can be measured. However, the resistance of the electrolytic portion does not change remarkably over time and the impedance of the device does not vary greatly. Thus, in the prior art the relation between the temperature of the device and impedance shown by the bold line of FIG. 19 has been maintained substantially constant, irrespective of a change over time.
However, once the durability of the aforementioned oxygen concentration detecting device has deteriorated, the correlation between the temperature of the device and the impedance resembles the dotted line shown in FIG. 19.
The structure of the air-fuel ratio sensor device, the equivalent circuit and impedance characteristics thereof will be described hereinafter.
FIG. 20 is a diagram showing the structure of the air-fuel ratio sensor device, FIG. 20A is a sectional view thereof and FIG. 20B is a partially enlarged view of an electrolytic portion thereof.
FIG. 21 is a diagram showing a circuit equivalent to the air-fuel ratio sensor. In FIG. 21, the code R1 denotes a bulk resistance of the electrolyte formed of, for example, zirconia (grain in FIG. 20); R2 denotes a grain boundary resistance of the electrolyte (grain boundary portion in FIG. 20); R3 denotes an interface resistance of an electrode formed of, for example, platinum; C2 designates a capacitive component of the grain boundary of the electrolyte; C3 designates a capacitive component of the electrode interface, and Z(W) designates impedance (Warburg Impedance) generated owing to periodic changes in the interface concentration upon polarization by an alternating current.
FIG. 22 is a diagram showing the impedance characteristics of the air-fuel ratio sensor device. The abscissa indicates a real part Z' of the impedance and the ordinate indicates an imaginary part Z". The impedance Z of the air-fuel ratio sensor device is expressed as Z=Z'+jZ". As FIG. 22 shows, it is evident that, as the frequency approaches the range of 1-10 KHz, the electrode interface resistance R3 converges to 0. A curve indicated by a broken line represents the change in the impedance upon deterioration of the air-fuel ratio sensor device. From the part of the impedance characteristic indicated by the broken line, it is evident that R3 varies as time elapses. When the oxygen concentration of gas detected by the air-fuel ratio sensor device sharply changes, the impedance characteristic varies as indicated by the broken line.
FIG. 23 is a diagram showing a relation between the frequency of an alternating voltage applied to the air-fuel ratio sensor device and the impedance of the device. In FIG. 23, the abscissa of FIG. 22 is converted to frequency f and the ordinate is converted to impedance Zac. Referring to FIG. 22, it is evident that the impedance Zac converges to a predetermined value (R1+R2) in a range from about 1-10 KHz to 10 MHz in frequency and the impedance Zac is reduced at a frequency higher than 10 MHz so as to converge to R1. This fact indicates that a range from about 1-10 KHz to about 10 MHz in which the Zac becomes constant irrespective of the frequency, is preferable so that the impedance Zac may be detected in a stable condition. The curve indicated by the broken line shows the variation of R3 over time or an impedance obtained when applying an alternating current at a low frequency as far as it can be measured (less than 1 KHz). A degree of the deterioration of the air-fuel ratio sensor device can be derived from the impedance at the low frequency.
As shown by the broken line of FIG. 19, the correlation between the temperature of the oxygen concentration detecting device as the air-fuel ratio sensor device and an impedance in a range from about 1-10 KHz to about 10 MHz greatly varies after the device has been deteriorated in comparison with the device before deterioration.
However, according to Japanese Patent Application Laid-Open No. HEI 9-292364, as only the resistance of the air-fuel ratio sensor device, R1+R2, is measured, the change in the characteristic of the air-fuel ratio sensor device cannot be clarified. Therefore, if the heater energization control is continued while maintaining the device impedance Zac as a target value for controlling the device temperature at 30.OMEGA., the control device temperature is gradually increased after deterioration of the device up to, for example, 800.degree. C. Therefore, the device is excessively heated, thereby further accelerating the deterioration and reducing the service life of the device.
Further, if the change in the device temperature or device characteristic causes inaccurate calculation of the air-fuel ratio based on an output value of the air-fuel ratio sensor, engine emissions may deteriorate. Likewise the fault or activation of the air-fuel ratio sensor cannot be determined accurately based on the device impedance detected in a state where the device temperature or device characteristics have changed.